Textbooks are essentially a collection of discrete sections of content.Take a chapter on, say, medical records in a doctor's office.
First comes an introductory section that provides an overview of the topic. From there will come a series of headings, each announcing what's to come in that section.
The key to writing that little introductory section is to write it first and last.
Here's what I mean.
Write It FirstTake a stab at what you'll be covering in the section to come. Then pay it no mind.
Write the rest of the section. Put in all your subheadings, bulleted lists, and whatever else is required to cover the content.
Then go back to the intro.
My guess is that your original sequence wasn't terribly close to the final sequence.
So by just throwing down an intro, planning all along to return to it, you'll free yourself to write the really good stuff. And after finishing the content section and revising the intro, you'll have the perfect setup for the reader.
So go forth! And then back again.